Crystalline ceramic compositions which have unit cells with superconducting planes are known and have been the subject of intensive investigation. An important general class of such crystalline ceramics is the metal oxide perovskites containing a lanthanide rare earth metal, an alkaline earth metal, and copper. One subclass of such compositions is represented by the formula RE.M.sub.2.Cu.sub.3 O.sub.7-x wherein RE is the lanthanide rare earth metal, M is the alkaline earth metal, and x is less than 1 and greater than 0. Another subclass contains lanthanum together with strontium or barium, and copper (viz. La-Sr-Cu-O or La-Ba-Cu-O). The type formula is represented as (La.sub.1-x Sr.sub.x).sub.2 CuO.sub.4, where x is less than 1 and greater than 0. See Bednorz, et al. (1986), Uchida, et al. (1987), and Hadika, et al. (1987).
Japanese researchers have investigated La-Sr-Cu-O ceramics, which comprise layered perovskites of the K.sub.2 NiF.sub.4 -type, including particularly the superconducting properties of La.sub.1.85 Sr.sub.0.15 CuO.sub.4. Yoshizaki, et al. (1987) and Hidaka, et al. (1987). Yoshizaki et al. prepared integrated samples of this composition by sintering and hot-pressing. They found that onset-temperature of the superconducting transition was almost the same for the polycrystalline sintered and hot-pressed samples. As reported by Hidaka et al., in polycrystalline specimens anisotropic electrical and superconducting properties could not be observed. These properties were necessarily studied in single crystals.
The most technically advanced superconducting ceramics are Y-Ba-Cu-O compounds, containing yttrium, barium, copper, and oxygen. Wu, et al. (1987) reported the type formula as (Y.sub.1-x Ba.sub.x).sub.2 CuO.sub.4-.delta.. For example, in a solid-state reaction, Y.sub.2 O.sub.3, BaCO.sub.3 and CuO were combined in proportions so that x was 0.4. The resulting ceramic specimens were superconductive at around 93K, well above the liquid-nitrogen boiling point of 77K. This could permit the required cooling to be obtained at greatly reduced cost. It was soon learned that the Wu, et al. specimens were a mixture of superconductive and insulating phases, and that the superconductive phase was represented by the formula YBa.sub.2 Cu.sub.3 O.sub.7-x (or YBa.sub.2 Cu.sub.3 O.sub.6+x). See Chu (1987 Preprint).
The present state of the superconductivity art has been currently reviewed in separate presentations by C. W. Chu and A. P. Malozemoff (1987 Preprints). Chu describes YBa.sub.2 Cu.sub.3 O.sub.7-x as representative of a more general class of superconductors having the general formula RE.Ba.sub.2 Cu.sub.3 O.sub.7-x, where RE is typically a lanthanide rare earth metal, and may include Y, La, Nd, Eu, Sm, Gd, Ho, Er or Lu. Chu states that superconductivity appears to be almost independent of the specific lanthanide. Superconductors of this general class have a unit cell with an elongated c axis, and provide a plurality of superconducting planes running perpendicular to the c axis.
Malozemoff (1987 Preprint) reported on further research with respect to YBa.sub.2 Cu.sub.3 O.sub.7-x. Superconductivity above 77K was detected in films formed by electron-beam deposition and plasma spraying, and single crystals were synthesized and shown to exhibit a high degree of anisotropy. A reference was made to then unpublished experiments of Chaduhri, et al., in which electron-beam-deposited Y-Ba-Cu-O films were found to form preferentially-oriented films. Malozemoff expressed the opinion that "new techniques" would need "to be devised to orient grains in applications incompatible with single-crystal substrates"